class Instrument():
    def make_sound(self):
        pass


class Erhu(Instrument):
    def make_sound(self):
        print('二胡在弹奏')


class Pinao(Instrument):
    def make_sound(self):
        print('钢琴在演奏')


class Violin():
    def make_sound(self):
        print('小提琴在演奏')


def play(obj):
    obj.make_sound()


if __name__ == '__main__':
    er = Erhu()
    pinao = Pinao()
    vio = Violin()
    play(er)
    play(pinao)
    play(vio)
